Preparation of finely divided acicular hexagonal ferrites having a high coercive force

ABSTRACT

A process for the preparation of finely divided acicular hexagonal ferrites which have a high coercive force and are of the general formula MFe 12  O 19 , where M is barium or strontium, and their use for the production of magnetic recording media and plastoferrites.

The present invention relates to a process for the preparation of finelydivided acicular hexagonal ferrites which have a high coercive force andare of the general formula MFe₁₂ O₁₉, where M is barium or strontium.

For a number of applications in the field of forgery-proof coding, forexample for identity cards, credit cards, and the magnetic storage ofother characteristic data, it is desirable to have magnetic recordingmedia whose coercive force is higher than that of the current standardstorage media. Appropriate materials would be less insensitive to straymagnetic fields and hence more difficult to forge.

Furthermore, according to Utility Model No. 82 33 253 plastoferritematerials having a very high coercive force are required for theproduction of print-through-echo-erasing means in tape cassettes, inorder to increase the signal to print-through ratio of magnetic tapes.

Hexagonal ferrites of the general formula MFe₁₂ O₁₉, where M is bariumor strontium, are usually used for this purpose.

Ferrite powders for the production of substantially forgery-proofmagnetic recordings are usually prepared by a ceramic method. For thispurpose, barium carbonate or strontium carbonate and iron oxide aremixed in a ratio corresponding to the chemical formula of the subsequentferrite, and this mixture is subjected to a heat treatment, i.e.pre-sintering, at from 1,100° to 1,300° C., which results in theformation of the magnetic hexaferrite. The sintered crystalliteagglomerates formed are then milled to a powder having a particle sizeof about 1 μm, milling generally being carried out with the addition ofwater. Milling produces crystal defects in the particles, and thesedefects result in a decrease in the coercive force. Ferrite powdersprepared in this manner generally have a very good specific remanentmagnetization, but the coercive force H_(c) is very low, being about 200kA/m before milling and less than 150 kA/m after the milling procedure.These crystal defects caused by milling can be only partially repairedby heating after milling, or by a sintering process, coercive forces ashigh as 300 kA/m being obtained. However, the disadvantage of thissubsequent heating is that the pigment becomes much coarser. If a coarsebarium ferrite powder obtained in this manner by milling and subsequentheating is incorporated into a plastic melt so that the degree offilling is high, the result is a very sharp decrease in the coerciveforce owing to the kneading process required.

Hexagonal ferrite powders are also prepared by conventional flux methodsin which fluxes, eg. B₂ O₃, alkali metal borates, PbO, alkali metalferrites, Bi₂ O₃, molybdates, alkali metal halides and alkali metalsulfates, are used to promote the reaction between the individual metaloxides. For example, according to U.S. Pat. No. 3,093,589, bariumferrite is prepared by heating a mixture of BaCO₃, acicular α-FeOOH andfrom 0.1 to 1% by weight of barium chloride, which acts as a catalyst,at from 890° to 980° C. Irregular tabular crystals with straight edgesare obtained. U.S. Pat. No. 3,793,443 describes a process for thepreparation of BaFe₁₂ O₁₉ powder from a mixture of BaCO₃, FeOOH and analkali metal chloride by heating at 1,000° C. After the alkali metalhalide has been washed out, regular tabular hexagonal crystals areobtained. U.S. Pat. No. 3,903,228 discloses a process in which ahomogeneous mixture of BaCO₃ and finely divided acicular α-Fe₂ O₃ havinga specific surface area greater than 20 m² /g is heated with from 3 to10% by weight of NaF at from 950° to 1,100° C. After washing, tabularcrystals having a diameter of less than 1 μm are obtained. According toU.S. Pat. No. 4,042,516, heating a mixture of 1 mole of SrCO₃, 6 molesof acicular α-FeOOH and from 0.05 to 2 moles of SrCl₂ at 1,000°0 C.,followed by extraction with water, gives tabular Sr ferrite having aplatelet size of about 2 μm. The flux methods have the greatdisadvantage that the products obtained in the heat treatment generallyhave to be freed from the catalytic flux by a process which involveswashing with water or dilute acids. Moreover, the added fluxes aregenerally highly corrosive and can both damage the crucible materialused and, because they are very volatile, destroy the furnace employed.

Processes for the preparation of barium ferrite, in which the startingmaterial used is a specific iron oxide hydroxide powder and no catalyticfluxes are added, have also been disclosed. For example, GermanPublished Application DAS No. 1,911,318 describes a process for thepreparation of magnetically anisotropic permanent magnets by subjectinga molding consisting of BaCO₃ and acicular α- or γ-FeOOH to a singleheat treatment at from 1,190° to 1,300° C., in which process the FeOOHneedles are mechanically aligned by the compression-molding procedure atright angles to the direction in which the pressure is applied. U.S.Pat. No. 3,723,587 describes a two-stage process for the preparation ofmagnetically anisotropic permanent magnets, in which a mixture of BaCO₃and acicular α-FeOOH is premolded and the resulting molding is preheatedat 1,000° C. and then compression-molded under a pressure greater thanthat employed in the premolding step. Further heating is then carriedout at 1,250° C. Japanese Pat. No. 5 4142-198 describes a process forthe preparation of tabular Ba ferrite and Sr ferrite, in which thestarting material used is a filter cake consisting of oriented BaCO₃ orSrCO₃ and acicular α-FeOOH particles. In this process, a suspension ofBaCO₃ or SrCO₃ and acicular α-FeOOH is filtered in a special manner,e.g. by means of gravity filtration, so that in the resulting filtercake the FeOOH needles lie parallel to its surface and are embedded infinely divided BaCO₃ or SrCO₃. The resulting filter cake, in itsentirety in uncomminuted form, or in the form of 10-20 mm cubes, isheated at 1,100° C., the FeOOH needles reacting with BaCO₃ or SrCO₃ andcoalescing to form tabular hexagonal ferrite having a particle diameterof from 0.5 to 1.5 μm. The sintered product obtained is then pulverized.The resulting barium ferrite powders have a coercive force of 200 kA/m,and the strontium ferrite powders obtained have a coercive force of 223kA/m. R. Takada et al. (Proc. Intern. Conf. on Ferrites, July 1970,Japan, pages 275-278) have examined, with the aid of an electronmicroscope, SrFe₁₂ O₁₉ permanent magnets produced by subjecting moldingsconsisting of SrCO₃ and acicular α-FeOOH to a single sintering treatmentat from 1,200° to 1,300° C. In this procedure, Sr ferrite formationtakes place topotactically in the moldings, the crystallographic (100)plane of the α-FeOOH being converted to the (0001 ) plane of the SrFe₁₂O₁₉. L. Girada et al. (J. de Physique, C1, Suppl. 4, 38 (1977), pageC1-325) were the first to obtain an Sr ferrite powder having a highcoercive force (H_(c) =446 kA/m) and a specific surface area of from 3to 4 m² /g by reacting a bulk powder consisting of a mixture of SrCO₃and acicular α-FeOOH, having a specific surface area of 13.1 m² /g, at1,050° C. in a high-temperature fluidized bed. The authors attribute thecomparatively low reaction temperature required (1,050° C.), theresulting fineness of the pigments, the narrow particle sizedistribution obtained and the consequent high coercive force to theelaborate high-temperature fluidized bed technique employed.

It is an object of the present invention to provide an economicalprocess for the preparation of a magnetic material which meets thedemands made on a magnetic material intended for use incounterfeit-proof magnetic recording media and in plastoferritematerials. Such a material should, in particular, exhibit gooddispersibility to enable it to be incorporated into organic binders, bevery finely divided and have a narrow particle size distribution, andpossess a high coercive force.

We have found that this object is achieved, and that, surprisingly,ferrites MFe₁₂ O₁₉, in which M is Ba or Sr, and which have the requiredproperties and are acicular in shape, can be prepared in a simple mannerif a neutral aqueous dispersion of acicular iron(III) oxide hydroxide isreacted with an aqueous barium chloride or strontium chloride solutionand a sodium carbonate solution, the resulting mixture is heated, thesolid phase of the resulting dispersion is separated off from theaqueous phase, washed thoroughly, dried and comminuted, and theresulting powder is heated at from 800° to 1,070° C.

Suitable acicular iron(III) oxide hydroxides are α-FeOOH and, inparticular γ-FeOOH. These powders are particularly useful for the novelprocess if they have a specific surface area of from 15 to 80,preferably from 20 to 50, m² /g, and the length/width ratio of theparticles is from 2 to 30:1, preferably from 5 to 25:1. The preparationof these iron(III) oxide hydroxides is known. Thus, α-FeOOH can beobtained by, for example, the procedure disclosed in German Laid-OpenApplication DOS No. 1,592,398, while γ-FeOOH can be prepared asdescribed in German Published Application DAS No. 1,061,760, German Pat.No. 1,223,352 or German Laid-Open Application DOS No. 2,212,435.

To carry out the novel process, an aqueous solution of the MCl₂ and anaqueous sodium carbonate solution are added to the stirred aqueousdispersion of the acicular FeOOH. The molar Fe/M ratio is advantageouslyfrom 9 to 12, and the molar Na/M ratio is advantageously from 2 to 4.The reaction mixture is then heated at from 60 to 100° C. for from 0.5to 3 hours, after which it is cooled, and the solid phase of theresulting aqueous dispersion is separated off from the aqueous phase,this usually being effected by filtration, and is washed chloride-freewith water and dried. Comminution of the resulting dry material to aparticle size of from 0.1 to 5, preferably from 0.1 to 1, mm is achievedby means of dry milling followed by screening. The pulverized drymaterial is then heated for from 0.5 to 3 hours at from 800° to 1,070°C. The material obtained is a member of the group consisting of thehexagonal ferrites, and is of the formula MFe₁₂ O₁₉, where M is Ba orSr.

The process according to the invention gives these ferrites directly inthe form of finely divided, unsintered powders. They consist of verysmall acicular crystals which essentially have the crystal shape of theacicular FeOOH employed and possess a specific surface area greater than5 m² /g. If acicular α-FeOOH is used as the starting material, theferrites prepared according to the invention have an unexpectedly highcoercive force, this being 420 kA/m in the case of BaFe₁₂ O₁₉ and 520kA/m in the case of SrFe₁₂ O₁₉. This value exceeds, by more than 70kA/m, the H_(c) value for SrFe₁₂ O₁₉ achieved by L. Giarda using thefluidized bed method. Moreover, we have found, surprisingly, that theuse of acicular γ-FeOOH as a starting material results in hexagonalferrite pigments having a still higher coercive force. For example, aH_(c) value of 450 kA/m is measured in the case of BaFe₁₂ O₁₉, whileSrFe₁₂ O₁₉ gives a value as high as 540 kA/m. Particularly advantageousis the good dispersibility of the novel ferrite pigments in organicbinders, resins or polymers.

Furthermore, the process according to the invention differs from theconventional flux method in that it is possible to dispense with the useof corrosive fluxes, which also have to be subsequently washed outagain. Moreover, the FeOOH needles can be coated uniformly with veryfinely divided MCO₃, where M is Ba or Sr, in a precipitation processwhich precedes the heating step. The advantage of coating the FeOOHneedles is that short diffusion paths are provided for the subsequentsolid-state reaction. This in turn results in lower reactiontemperatures and better retention of the acicular shape than can beachieved with conventional milling of the starting components.

Another advantage of the novel process is that the dry mixture of MCO₃and acicular FeOOH is used in comminuted form in the heating procedure.Hence, the novel process does not require any complicated compacting ortabletting techniques or any special filtration techniques, as arenecessary for the preparation of moldings with mechanical orientation ofthe FeOOH needles.

Furthermore, the novel process does not require any special, elaboratesintering techniques, such as a high-temperature fluidized bedtechnique, for heating the mixture of MCO₃ and FeOOH. Instead, heatingis carried out by a conventional method, for example in crucibles,dishes or tubular rotary furnaces. The surprising advantage of this isthat the reaction temperatures in the novel process are lower than thosein the conventional heating procedure.

The advantageous particle properties result in both good mechanicalproperties and improved dispersion behavior in organic binders, resinsand polymers. Because of this good dispersibility, the ferrite pigmentsprepared according to the invention are particularly useful as magneticmaterial in the production of magnetic recording media and of magneticplastoferrites. The ferrite powders prepared according to the inventionare so finely divided that the magnetic layers prepared therewithexhibit an outstandingly smooth surface even in the case of thin layers.Furthermore, compared with conventional, coarser ferrite products, thefinely divided ferrite powders prepared according to the inventionexhibit improved magnetic properties in magnetic recording media and inplastoferrites. For example, when the hexagonal ferrite powders preparedaccording to the invention are incorporated into an organic matrix,their high coercive forces are fully retained, so that magneticrecording media and magnetic plastoferrite materials having very highcoercive forces of up to about 550 kA/m can be produced. Tape-coatingexperiments to produce magnetic recording tapes have shown that thecoercive force can even increase by as much as 30 kA/m when a ferritepowder prepared according to the invention is incorporated into anorganic binder. By contrast, the coercive force of conventional, coarserferrite powders prepared by a ceramic method decreases very sharply whenthey are incorporated into an organic matrix material under similardispersing conditions. This is due to mechanical destruction of theferrite particles.

Because of the high H_(c) values of the ferrite/polymer compositematerials which can be prepared according to the invention, it isdifficult to alter a magnetic recording or permanent magnetization onceit has been effected, so that a stable permanent magnetic state isachieved. Hence, magnetic recordings in magnetic recording media aresubstantially insensitive to stray fields and are consequentlysubstantially forgery-proof. In the case of print-through-echo-erasingmeans, which can advantageously be manufactured with the plastoferriteparticles prepared according to the invention, the permanent magneticstate usually produced by magnetization by a discharge capacitor using ahigh field strength is completely unaffected by the demagnetizing fieldsproduced by the magnetic tape running past these means. Hence, when theecho-erasing means manufactured according to the invention fromplastoferrite material having a high coercive force are employed, thesignal to print-through ratio is greatly improved.

The Examples which follow illustrate the invention.

EXAMPLE 1

A dispersion consisting of 50.00 kg of acicular α-FeOOH having aspecific surface area of 27 m² /g and a length/width ratio of 10:1 and500 l of water was prepared with vigorous stirring, and a solution of12.835 kg of BaCl₂.2H₂ O in 50 l of water was added. A solution of 7.167kg of Na₂ CO₃ in 40 l of water was then added with continued stirring,and the resulting dispersion was heated at 90° C. for 2 hours, whilestirring, after which it was cooled and the solid phase of thedispersion was filtered off by means of a conventional filter press,washed chloride-free with water and dried. The dry material obtained wasdry-milled in a mill and then passed through a sieve having a mesh sizeof 0.3 mm. The comminuted, screened powder was heated in a stainlesssteel dish for 1 hour at 1,000° C. and then cooled. The resulting BaFe₁₂O₁₉ preparation, which was shown by the X-ray pattern to be a singlephase, consisted of acicular ferrite particles having a specific surfacearea of 7.5 m² /g. The coercive force (H_(c)) was 423 kA/m, and thespecific remanent magnetization (M_(r) /ρ) was 42 nTm³ /g.

EXAMPLE 2

A dispersion consisting of 4.00 kg of acicular α-FeOOH having a specificsurface area of 32 m² /g and a length/width ratio of 10:1 and 40 l ofwater was prepared with vigorous stirring, and a solution of 1.121 kg ofSrCl₂.6H₂ O in 4 l of water was added. A solution of 0.5793 kg of Na₂CO₃ in 3 l of water was then added with continued stirring, and theresulting dispersion was heated at 90° C. for 2 hours, while stirring,after which it was cooled and the solid phase of the dispersion wasfiltered off, washed chloride-free with water and dried. The drymaterial obtained was dry-milled in a mortar and then passed through asieve having a mesh size of 0.3 mm. The comminuted, screened powder washeated in a stainless steel dish for 1 hour at 1,000° C. and thencooled. The resulting SrFe₁₂ O₁₉ preparation, which the X-ray patternshowed to be a single phase, consisted of acicular ferrite particleshaving a specific surface area of 8.5 m² /g. The coercive force was 521kA/m, and the specific remanent magnetization was 43 nTm³ /g.

EXAMPLE 3

A dispersion consisting of 4.00 kg of acicular γ-FeOOH having a specificsurface area of 32.8 m² /g and a length/width ratio of 20:1 and 60 l ofwater was prepared with the aid of an Ultra Turrax dispersing apparatusfrom Kotthoff, and a solution of 1.020 kg of BaCl₂.2H₂ O in 4 l of waterwas added. A solution of 0.575 kg of Na₂ CO₃ in 3 l of water was thenadded with continued stirring, and the resulting dispersion was furthertreated as described in Example 2. Heating for one hour at 970° C. gavea BaFe₁₂ O₁₉ preparation which was shown by the X-ray pattern to be asingle phase and consisted of acicular ferrite particles having aspecific surface area of 8.1 m² /g. The coercive force was 450 kA/m, andthe specific remanent magnetization was 42 nTm³ /g.

EXAMPLE 4

A dispersion consisting of 4.000 kg of acicular γ-FeOOH having aspecific surface area of 32.8 m² /g and a length/width ratio of 20:1 and60 l of water was prepared with the aid of an Ultra Turrax dispersingapparatus from Kotthoff, and a solution of 1.113 kg of SrCl₂.6H₂ O in 4l of water was added. A solution of 0.575 kg of Na₂ CO₃ in 3 l of waterwas then added with continued stirring, and the resulting dispersion wasfurther treated as described in Example 2. Heating for 1 hour at 950° C.gave an SrFe₁₂ O₁₉ preparation which was shown by the X-ray pattern tobe a single phase and consisted of acicular ferrite particles having aspecific surface area of 8.7 m² /g. The coercive force was 539 kA/m andthe specific remanent magnetization was 44 nTm³ /g.

EXAMPLE 5

In a PR 46 Ko kneader from Buss, without a discharge screw, 3.75 kg/h ofbarium ferrite powder from Example 1 were metered into a stream of 1.25kg/h of a nylon melt consisting of Ultramid A3 from BASF AG, stabilizedwith 200 ppm of CuI and 800 ppm of KI, at 280° C., and the mixture wascontinuously kneaded and extruded. The barium ferrite powder and thepolymer granules were fed into the Ko kneader by means of differentialmetering balances from K-tron Soder GmbH. The resulting plastoferritematerial was cooled and then milled using a cutter mill. The granuleswere injection molded using an Allrounder 200 injection molding machinefrom Arburg at a melt temperature of 305° C. and a mold temperature of80° C. to give small cylindrical plastoferrite rods having a diameter of1.8 mm and a length of 5 mm. The barium ferrite content of theplastoferrite rods was determined by ashing at 400° C., and found to be75% by weight. The coercive force was 421 kA/m and the residualmagnetization was 76 mT. A cylindrical rod was permanently magnetized atright angles to the cylinder axis in a magnetic field of 800 kA/m. Itwas fitted in a magnetic tape cassette so that the magnetic fieldproduced by the plastoferrite rod was at right angles to the magnetictape running past it at a distance of 0.8 mm. Theprint-through-echo-erasing means increased the signal to print-throughratio by 8 dB.

EXAMPLE 6

Using the procedure described in Example 5, plastoferrite rods wereprepared from 75% by weight of barium ferrite powder from Example 1 and25% by weight of Lupolen 5021 D high density polyethylene produced byBASF AG. The melt temperature in the Ko kneader was 190° C., while thatin the injection molding machine was 255° C. The coercive force of theinjection-molded magnetic rods was 420 kA/m, and the residualmagnetization was 67 mT. The print-through-echo-erasing means increasedthe signal to print-through ratio by 6 dB.

EXAMPLE 7

6 kg of a commercial synthetic barium ferrite having a coercive force of120 kA/m and a specific surface area of 1 m² /g were wet-milled for 15minutes, with the addition of 1 l of water, in a Molinex PE 5high-performance pulverizer from Netsch, with the aid of 15 kg of steelballs (diameter 2-3 mm), at 1,500 rpm. The solid phase of the resultingaqueous dispersion was filtered off and dried, and the finely dividedbarium ferrite product was then heated for 1 hour at 1,020° C. Thebarium ferrite powder obtained had an H_(c) value of 300 kA/m and aspecific surface area of 2.3 m² /g. It was then incorporated intoUltramid as described in Example 5, the degree of filling being 88% byweight.

The coercive force of the plastoferrite granules was 180 kA/m. Hence,when the ferrite powder prepared by the ceramic method is incorporatedinto plastic, the coercive force decreases by 40%.

We claim:
 1. A process for the preparation of a finely divided acicularhexagonal ferrite which has a high coercive force and is of the formulaMFe₁₂ O₁₉, where M is barium or strontium, which consists essentially ofreacting an aqueous dispersion of acicular iron(III) oxide hydroxidewith an aqueous MCl₂ solution and a sodium carbonate solution, theamount of Na and M in the solutions providing an Na/M ratio of 2 to 4,the resulting mixture is heated, the solid phase of the resultingdisperson is separated off from the aqueous phase, washed thoroughly,dried and comminuted, and the resulting powder is heated at from 800° to1,070° C.
 2. A process for the preparation of a finely divided acicularhexagonal ferrite which has a high coercive force and is of the formulaMFe₁₂ O₁₉, where M is barium or strontium, as set forth in claim 1,wherein the acicular iron(III) oxide hydroxide is α-FeOOH.
 3. A processfor the preparation of a finely divided acicular hexagonal ferrite whichhas a high coercive force and is of the formula MFe₁₂ O₁₉, where M isbarium or strontium, as set forth in claim 1, wherein the aciculariron(III) oxide hydroxide is γ-FeOOH.
 4. A process as claimed in claim 1wherein the acicular FeOOH employed has a specific surface area of from15 to 80 m² /g and a length/width ratio of from 2 to
 30. 5. A process asset forth in claim 1 wherein the thoroughly washed and dried solid phaseis subjected to heating in a comminuted form having a particle size offrom 0.1 to 5 mm.
 6. A process as set forth in claim 1 wherein thethoroughly washed and dried solid part is subjected to heating in acomminuted form having a particle size of from 0.1 to 1 mm.
 7. Theprocess of claim 2, wherein the acicular FeOOH employed has a specificsurface area of from 15 to 80 m² /g and a length/width ratio of from 2to
 30. 8. The process of claim 3, wherein the acicular FeOOH employedhas a specific surface area of from 15 to 80 m² /g and a length/widthratio of from 2 to
 30. 9. The process of claim 2, wherein the thoroughlywashed and dried solid phase is subjected to heating in a comminutedform having a particle size of from 0.1 to 5 mm.
 10. The process ofclaim 3, wherein the thoroughly washed and dried solid phase issubjected to heating in a comminuted form having a particle size of from0.1 to 5 mm.
 11. The process of claim 2, wherein the thoroughly washedand dried solid part is subjected to heating in a comminuted form havinga particle size of from 0.1 to 1 mm.
 12. The process of claim 3, whereinthe thoroughly washed and dried solid part is subjected to heating in acomminuted form having a particle size of from 0.1 to 1 mm.
 13. Theprocess of claim 1, wherein the Fe/M ratio is 9 to
 12. 14. The processof claim 2, wherein the Fe/M ratio is 9 to
 12. 15. The process of claim3, wherein the Fe/M ratio is 9 to 12.